The present invention generally relates to the production of corrugated cardboard, and more particularly to a novel and improved method for accurately applying an adhesive to the flutes of corrugated board centered on the flute crests, so that the flutes can be bonded to a face.
Typically, corrugated cardboard is formed by producing a corrugated sheet which is initially bonded along one side to a single face. Adhesive is then applied to the crests of the flutes remote from the single face by an applicator roll of a glue machine. Thereafter, a second face is applied to the adhesive on the flutes to produce a composite structure in which corrugations extend between and are bonded to spaced-apart faces.
In some instances, multiple-layer cardboard is produced in which more than one corrugated sheet is adhesively attached to additional faces so that, for example, a central flat face is bonded to a corrugated sheet on each side thereof, and outer flat faces are bonded to the sides of the two corrugated sheets remote from the central face.
The corrugated sheet is typically passed between a rider roll and an applicator roll to apply the adhesive to the flutes. The rider roll typically applies sufficient downward pressure to force the flute tips into contact with the applicator roll. This downward pressure causes compression or deformation of the flutes. The flutes enter the adhesive layer prior to being crushed against the applicator and often become overly wetted or saturated with adhesive due to the long dwell time. As a result, the flutes do not return to their original shape after being crushed. This permanent deformation of the flutes reduces the strength of the final cardboard.
It has been known in the art that glue machines can be run with the applicator roll operating at a lower speed than the web speed (speed at which the corrugated sheet passes between the applicator roll and the rider roll) in order to adjust glue weight. U.S. Pat. No. 6,602,546, incorporated herein by reference, discloses a method for operating a glue machine such that the applicator roll can be operated at speed lower than the web speed while still applying a uniformly thick glue line at the flute crests, minimizing glue application along the leading or trailing faces of the flutes.
Previously, it was believed that the applicator roll must always be operated at a lower speed than the web speed. However, it has been discovered, surprisingly and unexpectedly, that excellent glue weight control and reproducibility also can be achieved when the applicator roll is operated at a speed (surface linear speed) greater than the web speed, e.g. at least 105% of the web speed